56 TRANSIT OF VENUS, 1874. HONOLULU. 



back to him by the glasses of the telescopes. The intermittent character of 

 the motion of the artificial planet was not perceptible in the telescopes. 

 These, removed from their mountings, were laid on a table as near together 

 as convenient, some eight or nine feet from the ground. A roofing of rushes 

 protected them and the observers from the direct rays of the Sun. The land 

 between was flat and grassy: 



The exact dimensions of the important parts of the model were as follows : 



Length of each chord of the Sun's limb 4'99 inches. 



Radius of curvature of ditto 2 4'47 > 



Length of the base of the triangular opening 8^46 , , 



Height of ditto 2-94 , , 



Diameter of the planet (mean) 2'857 , , 



Length of the chord on the planet joining the two points 



that touch the limb of the Sun at the internal contacts ... I '455 , , 



When the planet was set back horizontally exactly one inch from the 

 position of perfect internal contact at ingress, the distance of the cusps was 

 2-250 inches. 



The horizontal movement of the planet was one inch in 270 S> 3 mean solar 

 seconds. 



A mean-time chronometer was always used for the model practice. 



The distance of the model from the object-glasses of the telescopes may be 



considered 



798^4 feet for the 6-inch instrument. 

 799-6 ,, 4^-inch ,, 



The practice with the model, at Honolulu as at Greenwich, demonstrated 

 that there was no material difference between the different observers' appre- 

 ciation of the exact moment of contact, no matter what telescopes were 

 employed, provided the power was 100 or more. By suitable experiments it 

 was found that the internal contact observed with the telescope was within a 

 small fraction of a second of time of true mechanical contact. 



As regards the instant of internal contact the appearances of the model 

 bore no resemblance to the phenomena of the actual transit of Venus. The 

 comparisons of the various observers at the model therefore possess no 

 interest. 



It is different, however, with the micrometer measures ; the circumstances 

 of which, as far as the eye could judge, were a perfect imitation of the actual 

 ingress of Vemis. 



The daily micrometer practice on the model was a serious rehearsal of 

 what was to be done during the actual transit; and, the records being 



